The present invention relates to an injection locking type or MOPA (Main Oscillator Power Amplifier) type of laser device.
An injection locking type of laser device in which seed laser light oscillated from an oscillator is amplified in an amplifier is conventionally known and disclosed in, for example, Japanese Patent Laid-open No. 2000-357838. FIG. 8 shows a block diagram of an injection locking type of laser device according to a prior art, and the prior art will be explained based on FIG. 8 hereinafter. In FIG. 8, an injection locking type of laser device 11 includes an oscillator 11A for oscillating seed laser light 21A with narrow-banded wavelength, and an amplifier 11B for amplifying the seed laser light 21A and emitting amplified laser light 21B.
The oscillator 11A causes discharge between oscillator electrodes 14A and 15A inside an oscillator chamber 12A in which a laser gas including fluorine and neon or helium as a buffer gas, and excites the laser gas to generate the seed laser light 21A. The seed laser light 21A thus occurring is incident on a band narrowing unit 30 placed at a rear of the oscillator chamber 12A (the left side of the paper surface in FIG. 8), and is expanded by prisms 32 and 32, and only predetermined wavelength is reflected at a grating 33. This is called band-narrowing.
The seed laser light 21A emitted from the oscillator 11A passes through a rear window 19B from an injection hole 45 of a concave mirror 36 with a hole of the amplifier 11B, and is incident on the amplifier chamber 12B containing a laser gas. Amplification discharge is caused between amplifier electrodes 14B and 15B synchronously with the seed laser light 21A in the amplifier chamber 12B. Thus, pulse output of the seed laser light 21A is amplified with center wavelength xcexc and spectral line width xcex94xcex that is the same meaning as a spectral band width xcex94xcex (they are called wavelength characteristics) being kept, and the seed laser light 21A passes through a window 17B and is emitted from around a convex mirror 37 as the amplified laser light 21B. The amplified laser light 21B is incident on an aligner 25 to be a light source for exposure. In the above Laid-open Patent, the explanation is made with an ArF excimer laser device, but the same explanation is also applied to KrF excimer laser devices and fluorine laser devices.
However, the following disadvantages exist in using the injection locking type of laser device 11 according to the aforementioned prior art as the light source for exposure of the aligner 25 such as a stepper. Specifically, during exposure, it is sometimes necessary to suspend discharge for a specified period of time to stop the laser oscillation in order to replace wafers or reticles. When the laser oscillation is stopped, optical components such as a prism 32 and the like are not irradiated with laser light, so that temperature of the optical components and gas around the optical component decreases.
As a result, at the time of resuming oscillation after the suspension, there arises the disadvantage that the amplified laser light 21B having the center wavelength xcexc, which is deviated from a target value, is emitted and resolution of exposure is deteriorated. Further, there arises the disadvantage that it takes time to carry out a control to improve a change in the wavelength characteristics, and during that period of time, exposure cannot be carried out, which reduces availability of the aligner 25.
Especially when the injection locking type of fluorine molecular laser device is used as a source for exposure, the wavelength of the amplified laser light 21B is short, and thus the optical material that can be used in the aligner 25 is limited, which makes it difficult to remove chromatic aberration according to the optical component. Therefore, it is necessary to conform the center wavelength xcexc of the amplified laser light 21B to the target value more strictly as compared with the case of the excimer laser device. Consequently, in the case of a fluorine molecular laser device, it is difficult to conform the wavelength characteristic to a target value after oscillation is resumed, thus causing the disadvantage that more time is required as compared with the excimer laser device.
The present invention is made in view of the above-described disadvantages, and its object is to provide an injection locking type or MOPA type of laser device capable of promptly obtaining stable output energy and a wavelength characteristic.
In order to attain the above-described object, the injection locking type or MOPA type of laser device according to the present invention has a structure including
an oscillator for exciting a laser gas by oscillator discharge and oscillating seed laser light with wavelength being band-narrowed by a band-narrowing unit,
an amplifier for amplifying the seed laser light by amplification discharge and emitting the amplified laser light, a first wavelength monitor for detecting at least a wavelength characteristic of the seed laser light, and
a laser controller for performing adjustment oscillation to contain the wavelength characteristic of the seed laser light within a predetermined allowable range, at a time of startup or when laser oscillation is suspended for a predetermined period of time or more.
According to the above structure, at the time of starting laser oscillation or when resuming the laser oscillation after suspending it, the wavelength characteristic of the seed laser light is within the predetermined allowable range, and therefore the wavelength characteristic of the amplified laser light is near a target value just after starting or resuming the oscillation, thus reducing time required for the wavelength control.
Further, in the laser device,
the laser controller may drive the band-narrowing unit to contain the wavelength characteristic of the seed laser light within the predetermined allowable range on an occasion of the adjustment oscillation.
According to the above structure, the band-narrowing unit is actually driven, and therefore the wavelength characteristic of the seed laser light to be oscillated can be surely and promptly contain within the allowable range.
In the laser device,
a second wavelength monitor for detecting at least a wavelength characteristic of the amplified laser light may be further included, and
the laser controller may carry out a control so that the amplified laser light oscillates in synchronization with the seed laser light on an occasion of the adjustment oscillation.
According to the above structure, since the amplified laser light is synchronized with the seed laser light just before startup or during the suspension, the amplified laser light oscillates in synchronization with the seed laser light from a time just after startup or resuming oscillation, and thus the amplified laser light with an improper wavelength characteristic is not emitted.
Further, in the laser device,
a delay circuit for setting a delay time from light emission of the seed laser light to a start of the amplification discharge may be further included, and
the laser controller may output a command to the delay circuit to change the delay time and oscillate the amplified laser light in synchronization with the seed laser light.
According to the above structure, the delay time is adjusted, and therefore the amplified laser light can be surely synchronized with the seed laser light.